1. Field of the Invention
The present invention relates to a method for determining the stability of arc welding, and an apparatus therefor, especially the method and apparatus for use in a consumable electrode gas shield arc welding.
2. Description of the Related Arts
In recent arc welding systems, the output control of a welding power source has been shifted from the one of a thyristor type to the one of an inverter type, so that the control speed has been increased from 300 Hz to 15-60 kHz, thereby to be 50-200 times faster than the prior apparatuses, and that waveform control of the welding current and welding voltage can be made. Consequently, an improvement in arc starting property, improvement in the welding stability during a high speed arc welding, reduction of the amount of spatters produced in arc welding, and the like are effected, so that the welding stability in arc welding is being improved effectively. However, not only a starting stage, which is a welding operation stage provided when the arc welding starts, is unstable, but also it is difficult to maintain a stable welding quality even in a steady-state stage, which is the welding operation stage performed immediately after the starting stage, because the welding condition is varied due to machining strain, thermal strain or the like. Therefore, the instability of arc welding was a bar to countermeasures which are contemplated to prevent the products of bad welding quality from being produced due to the instability of the welding conditions caused in an automatic welding line by means of a welding robot or the like, and a partially automatic welding line. In addition, the determination of the welding stability in the prior arc welding system was relying on a qualitative determination through a visual examination.
Accordingly, various countermeasures have been contemplated, and various methods for determining the welding stability have been proposed. For example, a Japanese Patent Publication for opposition No. 2-62017 discloses a method for calculating a short-circuit time, arc time, short-circuit average current, arc average current, arc average resistance and arc electric power for each period or cycle, and calculating standard deviations of them to indicate a uniformity of the arc condition, arc terminating frequency and arcing level to determine a weldability. Also, a Japanese Patent Publication for opposition No. 5-57070 proposes a method for detecting the short-circuit time and arc time for each period by measuring a welding voltage, and obtaining an average or a standard deviation of the short-circuit time and the average or standard deviation of the arc time, based on which a welding parameter is calculated, to determine the weldability on the basis of the welding parameter. Furthermore, a Japanese Patent Publication for opposition No. 6-53310 proposes a method for representing a rate of variation of heat applied to the base metal and the wire during the short-circuit is formed between them, by employing a standard deviation of at least one of the maximum short-circuit current, an average of the current during the short-circuit, an effective current during the short-circuit, and an electric power during the short-circuit, to determine the weldability on the basis of the standard deviation.
In a Japanese Patent Publication for opposition No. 7-2275, proposed is an arc welding monitoring apparatus which measures the welding current and welding voltage, and uses a method of moving average to calculate a moving average of the welding current on the basis of the measured data. Furthermore, proposed are properly selecting a calculation time period and a moving pitch for calculating the moving average, and providing a monitoring period so as to avoid the period immediate before the start of arc welding, and the period immediate before the termination of the welding. In order to perform an optimal control of CO.sub.2 or MAG welding system by setting optimal welding conditions automatically, an optimal control method for arc welding is proposed in Japanese Patent Publication for opposition No. 6-53309. More concretely, a parameter for detecting the weldability quantitatively (referred to as weldability parameter (W)) is calculated by a short-circuit time (Ts), arc time (Ta), average current in the short-circuit period (Is'ave), average current in the arc period (Ia'ave), average resistance in the arc period (Ra'ave), and electric power in the arc period (Pa), as W=(.sigma..sub.Ts .multidot..sigma..sub.Ta .multidot..sigma..sub.Is 'ave.multidot..sigma. .sub.Ia 'ave/K).multidot.(Ra'ave/Ri).multidot.(Pa/Pi), where .sigma..sub.Ts is the standard deviation of Ts, .sigma..sub.Ta is the standard deviation of Ta, .sigma. .sub.Is 'ave is the standard deviation of Is'ave, .sigma..sub.Ia 'ave is the standard deviation of Ia'ave, K is a product of .sigma..sub.Ts .multidot..sigma..sub.Ta .multidot..sigma. .sub.Is 'ave.multidot..sigma..sub.Ia 'ave under a reference welding condition, Ri is a regression of Ra'ave under the optimal condition, and Pi is a regression of Pa under the optimal condition. Then, the outputs of the welding power source and the feeding amount of the wire are controlled so as to minimize the weldability parameter.
According to the above-described methods for determining the welding conditions disclosed in the above-described prior publications, however, an error is likely caused in determining the welding stability. For example, the average current in the arc period, which is employed in the Publication No. 2-62017, is easily varied depending upon the arc time. In the Publication No. 5-57070, the average of the short-circuit time and the average of the arc time are multiplied by the constants, respectively, and the sum of them is employed as the welding stability parameter. The method in the Publication No. 6-53309 uses the average of data. And, the method in the Publication No. 6-53310 uses the standard deviation of data only in the short-circuit period, without using the data in the arc period. Although the publication No. 7-2275 proposes to use the method of moving average, the determination is made based on the average of data. Thus, the above-described prior methods for determining the stability of arc welding are unsatisfactory, and some of them take a relatively long time to be analyzed, so that it is difficult to maintain the stability of arc welding. According to the methods in the publication Nos. 2-62017 and 6-53309, the parameters to be used in determining the stability are many, and the methods of processing them are complicated, so that it will take a relatively long time to determine the welding stability, and a software for use in determining the same will be of a great volume, so that a large capacity of memory will be needed.